Electroplating bath and method



United States Patent Oflice 3,489,660 Patented Jan. 13, 1970 3,489,660ELECTROPLATING BATH AND METHOD Peter P. Semienko, Roslindale, and EmilToledo,

Brighton, Mass., assignors to Honeywell Inc., Minneapolis, Minn., acorporation of Delaware No Drawing. Filed Jan. 3, 1966, Ser. No. 517,944Int. Cl. C23b 5/08, 5/04, 3/06 US. Cl. 204-43 12 Claims ABSTRACT OF THEDISCLOSURE This invention relates to methods of electroplating nickelousthin magnetic films and more particularly to a novel method forelectroplating thin non-magnetostrictive nickel-iron films from a noveltype electrolyte comprised of nickel ammonium sulfate and nickelsulfamate and ferrous ammonium sulfate ingredients arranged to provide anovel control over magnetic properties as well as improving conveniencein selection of source ion materials and providing improved bathstability.

Thin magnetic films of nickel-rich alloys are commonly electroplated,especially for use in data processing apparatus. The magnetic propertiesof such films are commonly difiicult to control, especially for filmswhich must have controlled magnetostriction. Such films are presentlyplated in electrolytes which are relatively unstable; as well as beingsomewhat expensive and inconvenient to prepare. The present invention isdirected to providing an electroplating bath, for such nickelousmagnetic films, which alleviates these problems.

Some magnetic nickelous film, especially Permalloy (nickel-iron; usuallyin -82 proportions) present a kind of dilemma as to selection of thesource-ion ingredients. That is, simple compounds, such as simplesulfates, are relatively convenient to use as such ingredients beingavailable as high purity reagent grade solids and quite stable duringplating life, unfortunately yield a plated film, the micro-structure ofwhich exhibits a great deal of undesirable stress; conversely, mostcomplex sourceion compounds present relatively the opposite benefits anddrawbacks. For instance, in plating Permalloy, one may, for convenience,use sulfates of nickel and iron which are readily available in reliableforms (AR grade purity, etc.) and are stable in solution, but whichcharacteristically provide a high stress film. One the other hand, onemight use nickel and iron sulfamate to provide a low stress film, butwill find that these ingredients are relatively expensive andinconvenient to obtain as solids at a reliable quality and also are veryapt to be unstable in solution. Iron sulfamate is especially unstable;for instance the free iron therefrom oxidizes all too readily,especially at higher bath pH and temperature. The invention provides asolution to this dilemma by prescribing nickel ammonium sulfatesource-ion ingredients for electroplating nickel alloy films; especiallyin combination with ferrous ammonium sulfate for plating magneticPermalloy. This complex (double) salt is unusual in that it isconveniently available in high grade lots and provides a very stableelectrolyte.

In addition to the above-mentioned problems of convenience, stabilityandlow stress, plating thin magnetic films also presents some especiallyserious problems in controlling magnetic properties, such as coercivity,squareness, zero-magnetostriction, and the like. These characteristicsare, in part, commonly related to the microstructure of the plated film(e.g. how the nucleation sites are started, how they build up duringplating, etc.) and are often affected by small, otherwise trivial,changes in plating conditions, such as a shift in bath temperature, pH,specific gravity and the like. Since the novel nickel ammonium sulfateba-se electrolyte mentioned above exhibits some such problems, it hasbeen found that a nickel sulfamate additive thereto greatly improvescontrol over magnetic properties without impairing stability. Therefore,it is preferred, according to the invention, to replace some nickelammonium sulfate in the novel electrolyte with a major portion of nickelsulfamate. Nickel sulfamate can be so added in amounts which keep theplated alloy composition unchanged, but yet surprisingly provide uniquecontrol over the micro-structure and magnetic properties of the platedfilm. For instance, it has been found that varying the ratio of nickelsulfamate to nickel ammonium sulfate can have a pronounced control oversquareness in a plated film, yet without changing the alloy composition.This rather surprising effect will be recognized as highly advantageousby those skilled in the art of plating magnetic films for computermemories. More particularly, replacing some of the nickel ammoniumsulfate in a Permalloy plating bath with nickel sulfamate, according toa prescribed range of ratios, can yield a plated non-magnetostrictivealloy with high squareness where otherwise the same alloy would bemarkedly non-square-a critical change for certain applications.

An added advantage associated with replacing some nickel ammoniumsulfate with nickel sulfamate is improved bath stability. The use ofnickel sulfamate with nickel ammonium sulfate provides an advantageousmeans of including the desirable sulfamate ion in an electrolyte toachieve magnetic control advantages (e.g. over a purely sulfate bath)and the like, without the drawbacks of other prior art sulfamatesources. For instance, nickel sulfamate is cheaper and more readilyavailable in reliably pure grades than iron sulfamate and, mostimportant, is much more stable since it will not oxidize as readily;Whereas prior art sulfamate baths would typically have a pH of about3.0-3.6 and thus readily oxidize free iron (such as from ferroussulfamate, ferrous ammonium sulfate, etc.) complex sulfamated bathsaccording to the invention can be kept at about 2.22.4 pH and thus aremore stable. For instance, it has been found that with such sulfamatednickel ammonium sulfate baths oxidation of ferrous ammonium sulfatetherein is negligible over an extended bath life. Nickel sulfamate alsoimproves low temperature solubility of nickel in the novel ammoniumsulfate bath. Thus, it has been found advantageous according to theinvention to replace some nickel ammonium sulfate with a major portionof nickel sulfamate to improve stability and low temperature solubility;still keeping enough nickel ammonium sulfate present to prevent the bathfrom becoming unstable. It is preferred to keep enough nickel ammoniumsulfate present to prevent bath pH from arising above about 3 pH, sinceiron ammonium sulfate begins to oxidize at a higher pH.

The above objects and features of novelty, as well as others describedbelow and apparent to those skilled in the art, may be derived,according to one form of the invention, by providing a novel electrolytefor electroplating thin magnetic Permalloy films. This electrolyteconsists essentially of an aqueous solution of nickel ammonium sulfateand nickel sulfamate in major amounts and in a prescribed ratio,according to the magnetic properties, stability and solubility required,and including ferrous ammonium sulfate in a minor amount, sufiicient toderive the prescribed nickel-iron alloy at the prescribed platingconditions; plus other customary electroplating constituents.

To provide the above-mentioned novel features and advantages accordingto the present invention, an electroplating process and novelelectrolyte therefor will now be described for electroplating a thinmagnetic nickeliron film onto a moving Wire substrate continuously. Moreparticularly a small copper coated Wire will be drawn at a prescribedspeed through the novel Permalloy plating electrolyte under prescribedplating conditions, first having preferably been electropolished,preferably in a novel sulfamic acid bath. A plated wire may be sodevised having controlled magnetostriction, squareness, etc. to beapplicable for use in magnetic memories, such as associated with dataprocessing machines.

Prior to introduction, continuously, into the plating electrolyte, thewire substrate will have been prepared to exhibit prescribeddimensional, metallurgical and surface characteristics, such as having acoating of at least one-half mil copper polished to a prescribedsmoothness of about #12. (:2) (STM rating). A 8 mil wire (diameter)having a beryllium-copper core has been found satisfactory for this,being introduced at the beginning of a continuous magnetic plating lineby unwinding from a spool and drawn through each treating stationsuccessively at about 5 inches per minute. More preferably, the wire isfirst electrolytically drawn to remove imperfections on the surface ofthe raw beryllium-copper wire according to a novel treatment describedin the co-pending commonly assigned application Ser. No. 518,013 to P.Semien-ko and E. Toledo entitled Metal Treatment incorporated byreference herein. It is also preferred that this drawn wire then beprovided with the aforementioned copper coating according to a novelcopper plating process as described in a co-pending commonly assignedapplication Ser. No. 518,184 entitled Improved Copper Coating by Seminkoet al., the details of which are incorporated herein. Subsequent to suchwire forming and copper plating treatments the wire is polished to aprescribed smoothness, such as about #12 to provide the proper magneticcharacteristics when plated with a thin film. Preferably, this iseffected by elcctro-polishing in a sulfamic acid bath as described belowrelative to Table IV. Subsequent to this electro-polishing the wire maybe further continually advanced through a clean water rinse and thenceto a magnetic plating station Where it is advanced through theaforementioned novel electrolyte to provide a thin magnetic film of afew microns thereon.

The wire is thus continually advanced, as stated, through the magneticfilm electroplating station, being drawn through the novel electrolytetherein as described below, such as using the bath and platingconditions recited in Table III. It will be presumed that, besides theplating described below, the plating will be otherwise conducted asknown by those skilled in the art, for instance using soluble anodemeans and, preferably, noniretallic fluid distributing means todistribute electrolyte homogeneously about the wire substrate. Apreferred form of such distributing means accommodates high agitation,high current density conditions.

It was found that the novel electroplating electrolyte may take a numberof forms. For instance, as noted in Table I, a number of nickel-richalloys (over 75% nickel) may be electroplated with improved results fromnovel ammonium sulfate-containing electrolytes. For instance, as notedfor baths A, B, C, and D, ammonium sulfate source ingredients mayconveniently provide the indicated plated films comprised of nickel,nickel-iron, nickel-cobalt and nickel-iron-cobalt, respectively. Filmswere plated from baths A, B and C in batch processes and from bath Dcontinuously on 5 mil wire. Several improvements were noted as regardsthese plating baths and the results derived therefrom. The baths werefound to be stable over a long period of time, for instance, maintaininga prescribed accurate composition over periods of upwards from aboutnine months, without requiring addition agents to adjust thecomposition, pH, etc. thereof.

TABLE I Bath Parameter Code A B C I) (1) Nickel ammonium sulfate, gm.250 250 250 250 (2) Nickel sulfamate (82 oz./gal.), L.- (3) Ferrousammonium sulfate, gm- 0 20 0 8 (4) Cobalt ammonium sulfate, gm--- 0 0100 8 (5) Saccharin, gm 0.4 .4 4 4 (6) Sodium lauryl sulfate, gm. 0. 1 11 1 (7) Boric acid, gm 6O 60 (8) Water, L To 1.0 1. l) 1. 0 1. 0 L, (9)Bath temperature, 0-- 90 90 90 (10) pH 2. 2 2. 2 2. 3 2. 3 (11) Currentdensity (ma/cm exemplar -400 80-400 80-400 80-400 Batch Batch BatchContinuous Plated alloy Ni 50 N1, 50 Fe 80 Co, 20 Ni 78 Ni, 18 Fe, 4 0

TABLE II.(PARAMETER CODE AS TABLE I) 1 2 3 4 5 6 T1 T2 T3 T4 340 360 430450 750 800 300 135 75 1. 8 1. 6 1. 7 1. 8 1. 6 1. 6 0 300 56 53 53 5452 45 8. 5 7. 0 7. 0 8. 5 20 20 23 23 22 30 4 4 4 4 1.6 1.6 1.8 1.8 2.00.5 .1 .1 .1 .1 288 320 330 330 405 450 70 45 to 60 60 6O 6.4 6.4 6.46.4 6.4 6.4 1.0 1.0 1.0 1.0

Max. 85 0., Pref. to 69 C. 85 65-70 C.

2.0-2.8 (Pref; 2.2-2.4) 2.0 2. 2 2. 2 2. 3 11 400-500 350 350 350 350Control of magnetostriction Average Better Best Best For plating theabove-mentioned 8020 nickel-iron thin magnetic films, however, it ispreferred to use modifications of the general bath indicated in Table I,namely baths 1 through 6 indicated schematically in Table II wherein aportion of the nickel ammonium sulfate source ion ingredients arereplaced by a prescribed major amount of nickel sulfamate asaforementioned. The plating con ditions obtaining in Table II will bepresumed to be those above-mentioned for continuously electroplating acOpper-surfaced wire of about 58 mils, advanced continuously at about 5inches per minute etc. Baths 1 through 6 are preferred, bath #6 beingthe most preferred and bath #1 the least preferred, while baths T1through T4 are somewhat more experimental. Baths 1 through 6 exhibitedan extremely high degree of stability requiring minimal adjustmentsthereof over a 9 month period. for instance, one parameter of stabilityis oxidation of iron ions detected by analysis for free Fe for instance.Fe++ was found free and unoxidized even after several months use ofthese baths, as opposed to what has been noted with prior art sulfamatebaths, wherein the Fe++ would become oxidized after just a few days use.More particularly, bath #1 required somewhat more frequent adjustments(e.g. because a Slight positive or negative magnetostriction was notedon the plated film). Such minor stability shifts may be caused, forexample by slight evaporation or by plating out of the source ionconstituents, thus modifying the operating (plating) range of the bath.However, all of these baths can tolerate relatively wide changes inconcentration of iron and nickel ions and are also considerably lesssensitive to changes in temperature, specific gravity (evaporation) andratio of iron to nickel than prior art baths. Using baths 1 to 6,relatively-stress-free magnetic thin films of about 81 nickel/ 19iron(:0.1%) and having practically zero magnetostriction were derived. Afeature of these sulfamate-ammonium sulfate baths was that theypermitted platin gat a relatively cool bath temperature (vs. pureammonium sulfate baths), namely about 69 C. and also plated relativelyquickly, using reasonable high current density levels. The low optimumpH range thereof (2.22.4) also improved stability and prevented anysignificant oxidation of ferrous source-ions. For baths 1 through 6 thesubstrate wire was 5 to 6 mils diameter (5 mil were electrolyticallydrawn to about 4.0 mils and having about 2 microns copper electroplatedthereon); the magnetic film being plated thereon to thicknesses from0.75 to 1.55 microns (115%).

The preferred conditions for plating thin Permalloy magnetic films asindicated above, using the af rementioned nickel ammonium sulfate/nickelsulfamate electrolytes are summarized in Table III below including theranges and preferred conditions therefor.

Table III-Bath ranges Nickel ammonium sulfate concentration-50-300 gm./1.

Nickel sulfamate concentrationsufficient for plated alloy at platingcondit. up to stability limit, e.g.: 100-300 gm./l. at 6570 C. 05 gm./l.at 85 C.

Nickel sulfamate for Perm no at Nickel Ammonium Sulfate- 8 N a y es-wof-19 (i0 1%) 1 260 ml. 0 mat about 0.

Ratio 1 4 2 Pref. e.g..for

Bath temperature (C.)-6570 for low magnetostriction pH about 2.0-2.8(pref. 2.2-2.4)max. 3.4 Plated thickness (microns)-0.52.0 (exemplary)78-82 Ni 81 Ni Plated alloys m prefer. T9178 (i0.1%)

With a controlled bath temperature (preferably close to 65 C.) aprescribed range of ratios of nickel sulfamate/nickel ammonium sulfatewill be required to control the alloy composition, such as to the zeromagnetostrictive alloy (81 nickel/19 iron). However, within this rangethese ratios may be adjusted to produce variations in microstructure,magnetic properties and the like, surprisingly with no change in alloycomposition. For instance, While a 1/4 ratio can yield this alloy, a 3/1 ratio can improve the fine smooth grain structure of the same alloyand also radically alter its squareness for magnetic purposes. As a ruleof thumb, the amount of nickel ammonium sulfate used may be establishedfor the solubility thereof at a prescribed bath temperature range, withnickel sulfamate added thereto to derive the required structure anddependent magnetic properties. And, of course, once the nickel ionconcentration has been selected this will dictate the amount of ferrousammonium sulfate (or other source ion ingredients) sufficient to provideiron of the proper amount in the plated alloy under the prescribedplating conditions, as known by those skilled in the art. Ferrousammonium sulfate is preferred because it is conveniently available inhigh grade, inexpensive forms. Ferrous ammonium sulfate is also morestable than other such ingredients, for instance, being much more stableand less subject to oxidation than ferrous sulfamate. This is especiallytrue at higher pH, such as at about 3, although some oxidation willoccur even with ferrous ammonium sulfate above 3.4 pH. Those skilled inthe art will also appreciate the advantages of operating at the lowindicated pH and low bath temperatures, while still maintaining areasonable plating efficiency.

As before mentioned it is preferred that the copper covered wiresubstrate be electro-polished in a sulfamic acid bath prior tointroduction to the electroplating station. Thus, for instance, thecopper plated wire aforementioned may be continually advanced through anelectropolishing bath where the copper finish may be finally smoothedbetween prescribed min/max. limits and also be sensitized for subsequentmagnetic plating in the aforementioned novel sulfate/sulfamateelectrolyte. This electro-polish may be performed preferably by asmoothing electrolyte such as indicated in Table IV (Examples I, II, andIII thereof) below. A novel sulfamic electro-polishing bath is providedaccording to the invention, both to polish more smoothly andefiiciently, and also to reduce contamination of the substrate forsubsequent plating. For instance, eliminating the sulfamic acidconstituent from a phosphoric acid polish bath has been found to inducethe formation of undesirable oxidation sites which will prevent platingthereon causing dropouts. Similarly, using sulfuric acid alone corrodesthe copper layer catastrophically, leaving intolerable discontinuitiestherein. The sulfamic type baths act to reduce the activity of thepolishing bath and inhibit post-copper-plating oxidation (which degradessubsequent magnetic plating). Thus, the sulfamic polishing baths providethe best control over plate-able surface finishing at a minimum loss ofplated copper thickness. For instance, they can produce a reproduciblesurface leveling of from 1 to 300 micro-inch RMS for dropout freemagnetic plating. The preferred electro-polishin'g conditions areindicated for Examples I-III below wherein it will be presumed that theabovementioned on-line wire treating conditions apply, such as advancingthe wire at about five-inches per minute and wherein the cell used isunderstood to include a cylindrical lead polishing cathode, as known inthe art.

(a) saccharin in a concentration of 0.4 to 4.0 g./l., (b) sodium laurylsulfate in a concentration of 0.1 to

TABLE IV Examples I II (Pref) III Range Bath:

Orthophosphoric acid 100 300 400 200-400 1111. Water 300 200 100 100-300ml. Sulfamic acid 1-15 (Pref. 6) 1-10 (Pref. 4) 1-5 (Pref. 2) 1-15 gm.(Pref. 2-6 gm.).

(B ath at room temperature, current density/time immersed, 2-50 Ina/cm.(Pref. 12); for about 50 sec.)

1 Under about 50 gin/L. water.

Sulfamic acid may be used up to the solubility limits of concentrationto maintain smoothness, but about gm. sulfamic acid per liter water ispreferred.

The above polishing steps have achieved a surpiising smoothness whenused with the copper plated wire aforementioned, reducing roughness apredetermined controlled amount for instance from #40 (STM smoothness;micro-inches, peak-to-peak) to as little as #1. Any desired smoothnesson the order of up to 3% of a typical plated thickness (about onemicron, i.e. 40 microinchcs) has been achieved. For instance, withExample III above, a current density of 50 rna./cm. will level a 4micron copper coating on 5 to 8 mil wire to about #4 STM roughness,reducing its thickness only about 1 micron.

The acid concentrations and other polishing conditions may be varied asunderstood by those skilled in the art. This electro-polishing step mayalso be applied to other metal (coated) substrates from the copperfamily, such as copper alloys, silver alloys etc. It is not applicablefor metals like nickel, iron and their alloys, however.

The copper-plated, electro-polished plated wire is now ready for use,e.g. to be provided with a magnetic thin film of controllableproperties, according to the invention. For instance, the wire may befurther continually ad vanced thrOllgh a following clean water rinse andthence to the above magnetic plating station for providing a thinmagnetic film of a few microns, by electroplating a nickeliron magneticfilm from the above sulfamate electrolyte.

It will be apparent to those skilled in the art that the principles ofthe present invention may be applied to different embodiments from thatshown; for instance to other types of substrates for improving themagnetic properties of films plated thereon. Likewise, the describedelectro-polishing step may be used to smooth other similar types ofsubstrates. While in accordance with the provisions of the statutes,there have been illustrated and described the best forms of theinvention known, it will be apparent to those skilled in the art thatchanges may be made in the condition of the processes disclosed withoutdeparting from the spirit of the invention as set forth in the appendedclaims and that, in some cases, certain features of the invention may beused to advantage without a corresponding use of other features.

Having now described the invention, what is claimed as new and for whichit is desired to secure Letters Patent 1. An aqueous bath for theelectrodeposition of an 8020 nickel iron magnetic information-storingalloy film, said bath (a) comprising nickel source material consistingessentially of 1) nickel ammonium sulfate in a concentration of 50 to300 g./l., and

(2) nickel sulfamate in a concentration of 100 to 300 g./l., with theweight ratio of said nickel source materials being between about 1 to 4and 4 to 1,

(b) comprising iron source material consisting essentially of ferrousammonium sulfate in a concentration of 7 to 9 g./l., and

(0) having a pH substantially between 2.0 and 2.8.

2. An aqueous bath as defineed in claim 1 further comprising 0.4 g./l.,and

(c) boric acid in a concentration of 30 to 70 g./l.

3. A method of electroplating a non-magnetic substrate with asubstantially -20 nickel-alloy magnetic infformation-storing film, saidmethod comprising the steps 0 (A) preparing an aqueous electrolyte fromiron source material and nickel source material in amounts to plate said'8020 nickel-iron film, with said iron source material consistingessentially of ferrous ammonium sulfate and with said nickel sourcematerial consisting essentially of nickel ammonium sulfate and nickelsulfamate, providing said nickel ammonium sulfate in an amount toprovide said bath with a pH not substantially above 3, and

(B) plating said film onto said substrate from said electrolyte.

4. A method as defined in claim 3 comprising the further step ofmaintaining said electrolyte at a temperature around 65 C. during saidplating step.

5. A method as defined in claim 3 comprising the further step ofmaintaining said electrolyte at a temperature substantially between 65C. and C. during said plating step.

6. A method as defined in claim 3 in which said electrolyte is preparedwith said nickel ammonium sulfate being present in an amount to maintainthe bath pH substantially between 2.2 and 2.4.

7. A method as defined in claim 3 in which said electrolyte is preparedwith the weight ratio of said nickel ammonium sulfate to said nickelsulfamate being between about 4 to 1 and 1 to 4.

8. A method as defined in claim 3 further characterized in that the filmis plated onto said substrate from said electrolyte with a currentdensity substantially between 300 and 600 milliamperes per squarecentimeter.

9. A method of preparing a plated wire memory element having an 80-20nickel-iron magnetic film plated onto a conductive copper bearingsurface, said method comprising the steps of (A) preparing an aqueousbath for electroplating said nickel-iron film from nickel sourcematerial and iron source material in amounts to plate said 80-20nickel-iron alloy therefrom, with said iron source material consistingessentially of ferrous ammonium sulfate and with said nickel sourcematerial consisting essentially of nickel sulfamate and nickel ammoniumsulfate with said nickel ammonium sulfate being present in an amount tomaintain the pH of said bath substantially between 2.0 and 2.8 and withthe Weight ratio of said nickel ammonium sulfate to said nickelsulfamate being substantially between 4 to 1 and 1 to 4, and

(B) electroplating said film onto said substrate from said electrolyte.

10. The method defined in claim 9 (A) comprising the further step ofmaintaining said electrolyte at a temperature substantially between 65C. and 85 C. during said plating, and

(B) in which said film is plated onto said substrate with a currentdensity between 300 and 600 milliamperes per square centimeter.

11. The method defined in claim 9 in which said bathpreparing stepincludes (A) introducing said nickel ammonium sulfate in a concentrationof 50 to 300 g./ 1., (B) introducing said nickel sulfamate in aconcentration of 100 to 300 g./l., and (C) introducing said ferrousammonium sulfate in a concentration of 7 to 9 g./l. 12. The methoddefined in claim 11 in which said bathpreparing step further includes(A) introducing saccharin in a concentration of 0.4

to 4.0 g./l., (B) introducing sodium lauryl sulfate in a concentrationof 0.1 to 0.4 g./l., and (C) introducing boric acid in a concentrationof 30 to References Cited UNITED STATES PATENTS 116,579 7/1871 Farmer204-49 XR 1,531,140 3/ 1925 Schneider 204-43 XR 2,254,161 8/1941 Waiteet a1. 204-49 OTHER REFERENCES Metal Finishing, p. 30, July 1962.Bartelson, B. I. et al.: Electrodeposition of Ni-Fe 10 Films, IBM Tech.Disclosure Bulletin, vol. 3, No. 2, p.

63, July 1960.

Berger, P.: Electrolytic Polishing of Brass Pressing, pp. 72-77, MetalFinishing, December 1948.

The Metal Industry, p. 5, Jan. 1, 1943.

JOHN H. MACK, Primary Examiner G. L. KAPLAN, Assistant Examiner US. Cl.X.R.

